Steam condensate corrosion protection is an important aspect of plant operation. An increase in the quantity and quality of condensate results in water and heat savings for the boiler industry.
The action of dissolved gases such as oxygen and carbon dioxide leads to condensate corrosion. When carbon dioxide dissolves, it reacts with water to form carbonic acid. The acid depresses the pH of the condensate and increases corrosion of the metallurgical parts in contact with the condensate. The corrosion may become manifest as a groove or the like found especially in steam headers or condensate return units. It can often weaken the pipe wall, especially at the threaded joints thereof, and such corrosion can also contribute to an increase in copper and iron that is returned to the boiler. Obviously, increases in metal levels in the boiler water can lead to troublesome deposit formation.
Oxygen, when present in the condensate, can cause a localized type of corrosion in the form of pitting. This type of corrosion can cause accelerated failure of the associated metallurgy due to localized concentration of this pitting problem in a small area of the condensate system.
Historically, three major approaches have been used to inhibit condensate system corrosion. Neutralizing amines have been used to neutralize the carbonic acid formed in the systems via control of system pH to the range of about 7.5-9.0. Morpholine, diethyaminoethanol, and cyclohexylamine are common neutralizing amines that are in use today. Typically, these neutralizing amines provide good protection against carbonic acid attack but offer little protection against oxygen based corrosion.
Filming amines, on the other hand, provide a thin protective barrier on the metallurgy in contact with the system condensate and protect the metal surfaces against both carbonic acid and oxygen attack. Additionally, a combination of the neutralizing and filming amines combines the elevated pH approach of the neutralizing amines and the film forming function of the filming amines to neutralize carbonic acid while providing a protective barrier film.
Steam condensate lines of boiler systems used in the food and beverage industry create special problems. Generally, these cannot be treated by conventional corrosion inhibition treatments due to regulatory prohibitions. Accordingly, corrosion inhibitors that were on the GRAS (generally recognized as safe) list were initially tested by our research group. From the GRAS inhibitors tested, lecithin was chosen for further development due to its performance and dispersability in water.
Lecithin is found in all living organisms, and the commercial grades are generally isolated from soybeans. Lecithin is a mixture of diglycerides of stearic, palmitic, and oleic acids, linked to the choline ester of phosphoric acid. Since it is a natural substance, lecithin is readily biodegradable and subject to rapid decomposition when under microbial attack. Microbial degradation of lecithin destroys the structure of the compound, and eliminates its protective corrosion inhibition capability in the steam condensate. Lecithin-based products, therefore, require preservation or they will not survive even short-term storage prior to use.
The application of a corrosion inhibitor to the steam condensate of a boiler servicing a food or beverage operation requires that the inhibitor, and all components of the inhibitor formulation, have approvals for that use by the U.S. Food and Drug Administration (FDA). A lecithin-based inhibitor, being a natural product, meets the FDA standards.
In most steam condensate operations (non-FDA), the application of preservatives to a corrosion inhibitor used to treat the condensate is unnecessary or easily accomplished with commercially available preservatives. This is not the case with a lecithin-based inhibitor that is adapted for usage in boiler systems that must comply with FDA regulations. The possibility of food contact requires that all of the inhibitor formulation components be FDA acceptable. The likely rapid biodegradation of the lecithin-based inhibitor during storage dictates that the inhibitor itself must be preserved, and the FDA requirement that all components of a product formulation carry FDA approval further dictates that the corrosion inhibitor must be preserved by a compound with FDA clearance for direct food contact.
In the development of the inhibitor, many known compounds (all FDA cleared) were evaluated as preservatives for lecithin. Among those tested were sorbates, benzoates, lactoferrin, sulfites, BHT, polysorbates, quaternary ammonium salts, isothiazolins, propionates, and other FDA approved preservatives. All of these were rejected because they were ineffective as preservatives in this application, destabilized the lecithin dispersion, or hindered the corrosion inhibition of the lecithin component.